How did Conifers grow in Mesozoic times? A Jurassic case of growth architecture in Araucariaceae from the La Matilde Formation, Patagonia, Argentina

Abstract

The architecture of a plant represents the expression of the equilibrium between endogenous growth processes and
environmental constraints. Simple morphological traits are used to describe a tree's architecture: the growth and
branching process, the morphological differentiation of axes, and the position of reproductive structures. The purpose
of this contribution is to describe the growth architecture of the Jurassic araucarian trees (in organic connection with
Araucaria mirabilis (Spegazzini) emend. Calder cones) present at the most basal part of the La Matilde Formation in
the locality of Monumento Natural Bosques Petrificados, Santa Cruz province, Argentina, and to compare them
with extant Araucariaceae. Both young and adult specimens had three branching categories from trunk (TA1) to
branchlets (A3), corresponding to the typical pattern in extant Araucariaceae. The main, orthotropic stem (TA1) had
rhythmic growth and branching, bearing plagiotropic branches in pseudowhorls. Main branches (A2) had rhythmic
branching with A3 in two orthostichies. Female cones were terminal at the tip of peduncles, lateral in position with
respect to A2 branches, peduncles had a position equivalent to A3 branches. Reiteration patterns are present: total
reiterations (bifurcated trunks and big orthotropic branches in trunks with normal plagiotropic branches) and partial
reiterations (epicormic branching). Two architectural "morphotypes" were recognized: the "Araucarioid morphotype
or morphotype A" includes most of the studied specimens; showing pseudowhorls of plagiotropic branches and
reiteration evidences; these characters correspond to Massart's model. "Morphotype B" was observed in bigger
trunks, with clearly orthotropic branches irregularly arranged, and no reiteration evidences; they could represent
Attim's model. Reiteration patterns are described for the first time for Jurassic gondwanic conifers, in agreement
with observations made in Triassic conifers from Arizona and Jurassic conifers from the United Kingdon. Moreover,
we can follow Massart's model from some Upper Palaeozoic Walchiaceae to the Mesozoic Araucariaceae studied in
this contribution.